Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/15—Nano-sized carbon materials
  • C01B32/182—Graphene
  • C01B32/184—Preparation
  • C01B32/19—Preparation by exfoliation
  • C01B32/192—Preparation by exfoliation starting from graphitic oxides
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
  • H05H1/00—Generating plasma; Handling plasma
  • H05H1/24—Generating plasma
  • H05H1/2406—Generating plasma using dielectric barrier discharges, i.e. with a dielectric interposed between the electrodes
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
  • H05H1/00—Generating plasma; Handling plasma
  • H05H1/24—Generating plasma
  • H05H1/2406—Generating plasma using dielectric barrier discharges, i.e. with a dielectric interposed between the electrodes
  • H05H1/2441—Generating plasma using dielectric barrier discharges, i.e. with a dielectric interposed between the electrodes characterised by the physical-chemical properties of the dielectric, e.g. porous dielectric
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
  • H05H1/00—Generating plasma; Handling plasma
  • H05H1/24—Generating plasma
  • H05H1/46—Generating plasma using applied electromagnetic fields, e.g. high frequency or microwave energy
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2004/00—Particle morphology
  • C01P2004/01—Particle morphology depicted by an image
  • C01P2004/03—Particle morphology depicted by an image obtained by SEM

Definitions

• the invention relates to a fast method applicable in industrial mass production of self- propagating process of reduction-exfoliation in porous material containing graphene oxide.
• Graphene has attracted significant scientific and technological attention for its remarkable electronic and thermal conductivity, large specific surface area, high chemical stability, and mechanical strength.
• various methods are used to prepare graphene, where graphene oxide (GO) reduction has great potential for mass production of graphene since GO can be produced from graphite on a large scale with cost-effective chemical methods.
• GO graphene oxide
• graphite is a hexagonal non-metallic mineral composed of atoms of carbon organized in a layered crystalline lattice. It is possible to produce graphene oxide (GO) from graphite.
• GO is a compound composed of carbon, oxygen and hydrogen in various ratios and is obtained by processing graphite by means of oxidizing agents and acids. This means that other atoms apart from carbon, especially atoms of oxygen, are introduced in the layered structure of graphite.
• GO is understood a thin sheet removed from the layered structure of graphite containing both, atoms of carbon and atoms of other elements. The decomposition of the layered structure to thin sheets is called exfoliation. The process of removing atoms of oxygen from the structure is called reduction.
• Graphene oxide consists of flakes composed of sp2 hybridized atoms of carbon of various sizes with different oxygen-containing groups attached to atoms of carbon.
• oxygen-containing functional groups present on the basal plane of a GO flake and the flake edge allow GO to interact with a broad range of organic and inorganic materials, but at the same time disrupt the conjugated aromatic graphene network and render GO flakes electrically insulating.
• the conductivity of GO necessary for many important applications can be dramatically increased by removing the oxygen-containing groups to form reduced multilayer graphene oxide (rGO) platelets with a related increased surface area and electric conductivity, which can be utilized as an alternative to graphene.
• rGO multilayer graphene oxide
• the partial restoration of the graphitic structure can be accomplished via thermal US 2007/0092432, chemical US 201703126951, microwave Han Hu, Carbon 50 (2012) 3267 -3273, laser US 8,883,042, and hydrogen plasma reduction US 8,182,917.
• the reduced graphene oxide (rGO) can be functionalized for use in different applications by, for example, treating rGO with other chemicals, by electric plasmas YIQING Wang et al.: J. Mater. Chem. A (2017) DOI: 10.1039/c7ta08607e, or by creating new compounds by combining rGO with other materials.
• the exfoliation was triggered by a local thermal shock due to a simple tapping a GO strip thin 10 - 20 pm with a hot soldering iron ( ⁇ 400 °C).
• a hot soldering iron ⁇ 400 °C
• GO has been investigated also as flame retardant material Nabipour H. et ah: Materials Chemistry and Physics 256 (2020) 123656 due to its intumescent behaviour when heated and its radical scavenging effects.
• Electric plasma is a reactive mixture of ions, electrons, and neutrals. It is generated by injecting sufficient energy into a working gas so that it becomes partly or fully ionized.
• the energy can be supplied for example, in the form of high electrical fields resulting in the so-called electron impact ionization, by heat, and by laser irradiation.
• the density of electrons and ions is nearly identical rendering the plasma as a whole nearly electrically neutral.
• the charged particles density must be high enough and the electrically neutral gas volume sufficiently filled by electrons and ions so that each particle can influence the nearby particles and thus generate collective effects. This is why the plasma is dominated by electric and/or magnetic forces.
• non-equilibrium electric plasmas generated by various types of electrical discharges can often be far from thermodynamic equilibrium. Therefore, desired chemical process can be conducted even under low working gas temperature conditions.
• the non-equilibrium plasma is attractive for many material processings including the plasma “reduction-exfoliation” of GO containing porous materials.
• the plasma treatment of GO may be easily performed in gas discharges generated at low pressures (say less than 0.1 atm) in still or flowing plasma working gases by immersing the treated porous GO containing material into a large volume of a uniform filament-less plasma K.
• Low-pressure gas discharge plasma processes are well understood and are used extensively in the semiconductor industry, the fact that vacuum conditions are necessary, makes the low-pressure plasma treatment impractical for high throughput and low-cost manufacturing of rGO.
• the low- pressure plasma treatment of graphene oxide also causes the destruction of the original shape of the material into fine particles of reduced graphene oxide.
• the treatment of GO containing materials using small plasma volumes results in limited and spatially controlled reduced-exfoliated volumes with increased conductivity and internal surface area that is useful, for example, to pattern the conductive rGO onto the GO paper and GO coated textiles Zheng Bo et ah: J. Phys. Chem. C 2014, 118, 13493-13502, V. Kedambaimoole et al.: ACS Appl. Mater. Interfaces 2020, 12, 15527-15537.
• the localized plasma treatment area and volume constitute significant limitation and technical problem particularly for the highly demanded atmospheric-pressure plasma-assisted high-throughput manufacturing of large area and volumes of free-standing porous materials containing graphene oxide reduced by means of plasma generated under atmospheric pressure.
• conductive and flame retardant textiles lightweight electromagnetic wave absorption materials, electrode materials for batteries and supercapacitors, materials for solar energy collection, catalyst supports, materials for adsorption removal of radionuclides, arsenic, antibiotics, bilirubin etc.
• the ionized plasma working gas can be produced also by heating the plasma working gas to become ionised at the plasma working gas temperature above say 800 °C as assumed by Chii-Rong Yang et al.: Nanomaterials 2018. 8, 802. Subsequently the ionized plasma working gas flows through a plasma nozzle at a speed higher than 1 m/s creating just behind the nozzle an electric plasma volume with the plasma flow towards the treated material localized in a low-electric-field region at a distance larger than 1 cm from the plasma nozzle.
• the long treatment time in the order of minutes is characteristic also for the atmospheric- pressure plasma reduction of porous materials containing graphene oxide immersed into volumetric filamentary plasmas of the so-called volume dielectric barrier discharges (DBDs) well known in the art
• DBDs volume dielectric barrier discharges
• Quan Zhou et ah J. Mater. Chem., 2012,22, 6061-6066, Yiqing Wang et al.: J. Mater. Chem. A 2017 DOI: 10.1039/c7ta08607e.
• the electric field strength inside of the porous material treated by the volume DBDs is higher than a critical field strength necessary for the gas ionization by electron impact, the plasma filaments enter the porous material volume with the high field causing inside localized thermal and mechanical damages.
• DCSBD Diffuse Coplanar Barrier Discharge
• the above described object of the invention is solved by creating a method of triggering a self- propagating reduction-exfoliation process of graphene oxide in a porous material containing graphene oxide according to the present invention.
• the method relates to triggering a self-propagating reduction-exfoliation process of graphene oxide in a porous material containing graphene oxide to increase electric conductivity and the specific surface area of the porous material.
• the subject matter of the invention consists in that the initial electric plasma is generated in the adjacent part and only partly in the inside part of the total volume of the reduced-exfoliated porous material.
• This triggers the self-propagating reduction-exfoliation process wherein to generate the initial electric plasma the parameters of the following group are fulfilled: the temperature of the working gas is less than 400 °C, the pressure of the working gas is higher than 10 kPa, the speed of the working gas is less than 0.1 mxs 1 , the temperature of the total volume of the porous material is less than 200 °C.
• the primary benefit of the invention consists in that the initial electric plasma is generated only in a part of the total volume, whereupon the invention takes advantage of the triggered self propagation of a hitherto unknown reduction-exfoliation process.
• the invention applies the local triggering of the hitherto unknown reduction-exfoliation process with an avalanche extension in the rest of the total volume for an industrial mass employment to modify porous materials containing graphene oxide.
• the hitherto unknown reduction-exfoliation process can be triggered by the electric discharge plasma reduction-exfoliation already known in the art.
• the discharge plasma reduction-exfoliation known in the art is due to bombardment of GO by energetic discharge plasma electrons present within a limited discharge plasma volume where local values of the so-called Laplacian electric field are higher than the so-called critical electric field strength necessary for the electron impact ionisation generating the energetic plasma electrons and specific for the plasma gas used.
• Laplacian electric field are determined by the geometry of discharge electrode system used to generate the plasma and electric voltage applied to the electrodes without the discharge plasma.
• the primary ionization process generating the plasma is due to electron impact ionization of working gas molecules when the electrons can gain sufficient energy within the mean free path from the electric field to cause ionization.
• the ionization energies for nitrogen and oxygen molecules are 15.5 and 12.2 eV
• the corresponding mean electron free paths at the atmospheric pressure are 6.28 and 6.79 pm respectively.
• a voltage of about 10 4 -10 5 V is required to cause the plasma-generating electric discharge for a 1 cm gas gap corresponding to the critical electric field about 10 -100 kV/cm.
• the critical electric field also termed the breakdown electric field, is specific for any plasma working gas, and in atmospheric air it is known to be 3.0 x 10 4 V/cm.
• the initiating plasma volume taking the form of a small (of ⁇ 1 mm radius) plasma plume can be generated also by an excimer or CO2 laser irradiation at the incident laser fluence above 10 J.cm 2 .
• the initial electric plasma is generated by means of dielectric barrier discharge, especially by a diffuse surface dielectric barrier discharge.
• Fig. 1A is a photograph of an initial electric plasma volume generated in the area of approximately 4.5 cm by 1.5 cm and the thickness of 0.3 mm generated by a coplanar surface dielectric barrier discharge in laboratory air in the volume where Laplacian electric field values are above the critical field strength above approximately 3.0 x 10 4 V/cm.
• the initial plasma volume is not well discernible in Fig. IB because of an intense external light necessary for taking the photograph.
• Figs. IE to 1G are photographs taken in different time and they show the reduction-exfoliation process occurring in the volume of the porous material where the Laplacian electric field values are below the critical field strength.
• Fig. 2 shows temporal development of the sample temperature in time during the plasma triggered reduction-exfoliation process illustrated by Figs. IB - ID.
• Fig. 3A is an image of an original aerogel sample of graphene oxide from a scanning electron microscope.
• Fig. 3B is an image from the scanning electron microscope of the original sample of graphene oxide after the reduction-exfoliation process triggered according to the invention that generated the reduced graphene oxide shown.
• Fig. 4 are photographs of a 3D self-standing structure of the samples of reduced graphene oxide prepared by plasma triggered reduction-exfoliation process illustrated by Figs. IB - ID.
• Fig. 5A is a so-called aerogel “cake” of graphene oxide situated on the surface of a commercial DCSCD electrode system.
• Fig. 5B is a so-called aerogel “cake” of reduced graphene oxide fabricated according the present invention by the plasma triggered reduction-exfoliation of the GO aerogel cake in nitrogen gas atmosphere at atmospheric pressure.
• Fig. 6 is sample of PP (polypropylene) nonwoven fabric coated by a thin, porous graphene oxide layer partly reduced-exfoliated according the present invention.
• Fig. 7 is a sample or reduced graphene oxide fabricated according the present invention using the volume DBD plasma triggered reduction-exfoliation of graphene oxide.
• Fig. 8 and 9 are schematic illustrations of the interception of the plan view of the graphene oxide sample and the initial volume of plasma.
• Fig. 10A and 10B are schematic illustrations of the sample placed in a device from the side view.
• the porous material to be treated according to the invention can have the structure of a powder layer, an open cell foam, GO paper or aerogel which can be reinforced with polymers, sponge and other free-standing structures, a non- woven fiber structure, or a woven fiber structure.
• porous refers to a GO containing material which is permeable such that fluids are movable therethrough by way of pores or other passages.
• content of GO in the treated material there is no particular lower limit to the content of GO in the treated material.
• the relative GO content can be very low if the material to be treated is a fiber structure consisting of relatively thick polymer fibers coated by a thin layer of GO.
• Fig. 1A taken at a low external light irradiation shows well discernible bright initial plasma volume 3 generated in the laboratory air volume above the DCSBD electrode system, such as depicted in ( M . Simor et al: Appl. Phys. Lett. 81, 2716 (2002) at voltage of 7.9 kV that is 50 % above the discharge onset voltage).
• the initial plasma volume 3 of a 0.3 mm thickness and 1.5 cm by 4.5 cm area was generated in the laboratory air volume, where the values of the Laplacian electric field were higher than the value of the critical electric field.
• Fig. IB shows a sample of self-standing GO aerogel of the thickness approximately 1 mm with its total volume 2_situated partly on the surface of DCSBD electrode system at the moment of the onset of the initial plasma volume 3.
• the initial plasma volume 2 is not discernible in FIG. IB because of an intense external light necessary for taking the photograph.
• the volumes 1_, 2 and 4 are schematically illustrated in Fig. 1C. Note that this is one example of many possible experimental arrangements and techniques generating the initial electric plasma volume 3 also inside a part 4 of the total volume 2 of the porous GO containing material to be reduced-exfoliated. As illustrated by Fig. 1C a part of the thin initial plasma volume 3 was intersecting with the part 4_of the volume of the sample, i.e., was in contact and penetrating vertically less than 0.3 mm into the volume 2. In this small part 4 where the local electric field values were higher than the critical value of 3.0 x 10 4 V/cm, the plasma reduction-exfoliation process known in the art took place.
• Figs. IE - 1G illustrate the hitherto unknown fast reduction-exfoliation process spontaneously propagating horizontally outside of the initial volume 3 of the initial electric plasma with a speed of approximately 10 cm/s.
• Fig. 1G shows the situation after the completed reduction-exfoliation of the full volume 2 of the sample. From the change of the sample colour from dark brown (GO) to black (reduced GO) it can be seen that the major part 1 of the total sample volume 2 was reduced-exfoliated not by the initial volume 3 of the initial electric plasma, but by a hitherto unknown process triggered by the initial reduction-exfoliation process in the part 4. It should be noted that this process took place also at the longitudinal distance of several millimeters from the boundary of the initiating volume 3 of the initial electric plasma, where the values of the Laplacian electric field strength determined from the electrode geometry and the applied voltage geometry, were far less than the critical electric field (i.e. the dielectric strength) necessary for the initial plasma formation due to the electron impact ionization.
• the critical electric field i.e. the dielectric strength
• Fig. 2 shows temporal development of the temperature of the sample measured using a contact thermocouple with the marked time of the plasma onset, as well as the times of the onset and completion of the reduction-exfoliation process determined by a video camera record. It is evident that during the hitherto unknown process reduction-exfoliation the sample temperature was less than 200 °C and that the process was completed within several seconds after its triggering by a plasma reduction-exfoliation in part 4 of the volume, wherein the electrical conductivity and porosity of the sample were increased by 10 8 and 3 folds, respectively. The change of the porosity and micromorphology of the sample due to the reduction-exfoliation process according to the present invention is shown in Figs. 3 A - 3B.
• the results according to the invention are very sensitive to the chemical composition of the treated GO containing porous material as, for example, to the GO content, content of trapped interlaminar water, the content of ammonium hydroxide often used to adjust the pH value of the GO water dispersion, and to sulfone groups bonded to GO when it was prepared using the modified Hummel method.
• an exemplary and non-limiting way is to use the so-called dielectric barrier discharges with different electrode geometries well known in the art to generate nonequilibrium plasmas at near-atmospheric gas pressures.
• the phrase “generating the initial volume 3 of the initial electric plasma partly inside the total volume as used herein refers also to the sequence when the initial volume 3 of the initial electric plasma is created outside the total volume 2 and subsequently contacted with the part 4 of the total volume 2 by, for example, a relative movement of the initial volume 3 of the initial electric plasma to the total volume 2 of the treated GO material.
• plasma gas temperature refers to the rotational temperature of the electrically neutral gas molecules in the plasma that has been used widely as gas temperature measurement in different types of electric plasmas and has been assumed to be in equilibrium with translational temperature of the gas molecules.
• initial electric plasma refers to a classical electric plasma where the following applies: proportions of the generated plasma are substantially larger than the so-called Debye length well known from the present electric plasma theory. As inferred from, for example ( Davide Mariotti and R Mohan Sankaran 2010 J. Phys. D: Appl. Phys. 43 323001), under the conditions of the present invention the Debye length is approximately on the order of 10 4 - 10 5 m.
• the method according to the present invention was used to reduce-exfoliate the graphene oxide sample identical to that shown in Figs. IB - 1C.
• the reduction-exfoliation process was triggered by an initial electric plasma generated by irradiating the sample by an intensive plume.
• the sample was at a room temperature of 22°C placed on the DCSBD electrode system similarly as shown in Fig. IB, but the voltage applied to the electrodes was of 3.1 kV, i.e. only 50% of the discharge onset voltage necessary to ignite the DCSBD and to generate the discharge plasma and, therefore no thin discharge plasma layer such as seen in Fig. 1A was generated.
• the part of the sample localized directly at DCSBD electrode system see Fig. IB, was irradiated by Q-Switched Nd:YAG laser (20 Hz, 1064 nm, 8 ns pulse width) at the incident laser fluence 15 J.cm 2 resulting in the formation of an initial volume 3 of the initial electric plasma there. Faser pulses were directed perpendicularly to the sample surface and focused to a spot of approximately 0.5 mm diameter.
• a GO aerogel cake of 5.5 cm diameter, 1.5 cm thickness of dark brown colour shown in Fig. 5A was fabricated under mild conditions from an aqueous solution of GO by drying in a vacuum oven at 60 °C for 24 hours.
• the GO cake was placed at a room temperature on the electrode system surface of a commercial initial DCSBD plasma source supplied by Roplass Ltd. (Brno, Czech Republic) in nitrogen gas atmosphere at atmospheric pressure.
• the initial plasma source energized by 8.6 kV alternating voltage generating electrical discharge of 90 W total plasma power.
• the fast plasma triggered reduction-exfoliation process according to the present invention was triggered by the initial DCSBD plasma in 2 s after the nitrogen plasma ignition and completed in next 2 s, as indicated by the black colour of the reduced-exfoliated GO material shown in Fig. 5B.
• the nitrogen-doped reduced GO fabricated by this method has a high carbon/oxygen ratio of 10 and a nitrogen content of 3 atom %.
• the conductivity of N-doped aerogel measured by Four-Point Probe Meter at ambient temperature with a value of 2.4 x 10 -2 S m _1 .
• the porous reduced-exfoliated GO fabricated via the method according to the present invention was pressed into the thin sheet of thickness 50 pm and subsequently analysed by Four- Point Probe Meter at ambient temperature with the conductivity value of 500 S.m 1 .
• a 2.50 cm x 4.5 cm sample of 15 gsm polypropylene spunbond nonwoven fabric was hydrophilized by a 0.5 s exposure to laboratory air DCSBD plasma.
• the initial plasma source was energized by 10.5 kV alternating voltage generating a thin 21 cm by 8.5 cm by 0.03 cm laboratory air plasma volume of 400 W total plasma power in laboratory of relative humidity 30 %.
• this thin initial plasma layer triggered the process of reduction-exfoliation according to the present invention resulting in the formation of the black conductive volume of PP fabric coated by the reduced GO outside of the 0.3 mm thick initial DCSBD plasma volume. This means the initial electric plasma extended only in 0.3 mm of the total thickness of 1 mm.
• the method according to the present invention was used to reduce-exfoliate the graphene oxide sample identical to that shown in Fig. 1 A and described in Example 1.
• the reduction-exfoliation process was triggered by an initial laboratory air plasma generated by a volume dielectric barrier discharge (DBD).
• DBD volume dielectric barrier discharge
• the lower electrode of the volume DBD was made from an aluminium plate.
• the upper optically transparent electrode was made from a glass Petri dish of a diameter of 8 cm filled with electrically conductive salty water.
• the discharge gap between the aluminium electrode and the Petri dish bottom was 1 mm.
• the volume of GO sample identical to that described in Example 1 was inserted partly in the gap between the electrodes and fixed in such position by a tape. Subsequently the initial volume 3 of the initial electric plasma marked by bright spots of thin plasma filaments seen in Fig. 7 was generated between the electrodes by application of 12 kV/10 kHz AC voltage.
• the method of self-propagating reduction-exfoliation of graphene oxide in a porous material containing graphene oxide to increase electrical conductivity and the specific surface area of the porous material created according to the invention is applicable e.g. in the development and production of electronical components, in chemical industry, in textile industry etc.

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